Define an instrument? A computer can be an instrument just as much as the violin or guitar so saying its made on a computer can never mean it isn't music.
I don't like a lot of rap music but at its roots and base it was a meaningful and lyrical expression of the life and times and there are many amazing examples of rap songs that would put much country, rock and heavy metal to shame.
Basically it may not be my type of music but it is music. Anytime he tries to say otherwise here are some responses.
1. made on a computer: Made on a block o wood with some string mostly controlled by a computer? no difference really both instruments.
2. lyrics? Country music is either about being drunk or about being rejected. Metal about self harm and Rock about Drugs. If you try to reduce a genre to its work and sadly most popular sub-genre of lyrical content then nothing looks good but Rap has more 'Real' lyrics then most of the crap in the charts.
As I said its not my usual taste but doesn't mean its not music and your friends arguments hold no water. Go back only a short time and The 'Real instruments' he is talking about would be considered crap toys. (I played the violin and the Base guitar btw.)